1. Field of the Invention
The present invention generally relates to methods and systems for identifying defect types on a wafer. Certain embodiments relate to a computer-implemented method that includes identifying defect types of defects detected on a wafer based on output acquired by a set of different combinations of illumination and collection channels of an inspection system.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etching, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Inspection for many different types of defects has also become more important recently. For instance, in order to use inspection results to monitor and correct semiconductor fabrication processes, it is often necessary to know what types of defects are present on a specimen. In addition, since controlling every process involved in semiconductor manufacturing is desirable to attain the highest yield possible, it is desirable to have the capability to detect the different types of defects that may result from many different semiconductor processes. The different types of defects that are to be detected may vary dramatically in their characteristics. For example, defects that may be desirable to detect during a semiconductor manufacturing process may include thickness variations, particulate defects, scratches, pattern defects such as missing pattern features or incorrectly sized pattern features, and many others having such disparate characteristics.
Classifying defects found on wafers and other specimens has, therefore, become increasingly important in order to determine what kinds of defects are present on the wafers in addition to distinguishing defect types of interest from other defect types. Several fully automatic defect classification (ADC) tools are now available. Typically, these tools use classification “recipes” to perform defect classification. A “recipe” can be generally defined as a set of instructions that define an operation to be performed by a tool and that are provided to and run on the tool upon request by a user. Classification recipes are typically generated using previously acquired data for specific defect classes that may be assembled in a suitable database. In the simplest implementation, the ADC tool can then compare unknown defects to those included in the specific defect classes to determine which defect class the unknown defect is most like. Obviously, much more complicated algorithms can be used by the ADC tool to determine the defect class to which the unknown defect most likely belongs.
Sometimes ADC is performed after inspection of a wafer. However, some systems and methods have been developed that can be used to perform ADC during inspection or “on-the-fly.” Examples of such systems and methods are illustrated in International Publication No. WO 99/67626 by Ravid et al., which is incorporated by reference as if fully set forth herein. The systems and methods described in this publication are generally configured to separately detect defects in the electrical signals produced by different detectors. In other words, the electrical signals produced by each of the detectors are processed separately to determine if each detector has detected a defect. At any time that a defect is detected in the electrical signals produced by one of the detectors, the electrical signals produced by all of the detectors are analyzed collectively to determine scattered light attributes of the defect such as reflected light intensity, reflected light volume, reflected light linearity, and reflected light asymmetry. The defect is then classified (e.g., as a pattern defect or a particle defect) based on these attributes.
Other currently available inspection systems are configured to inspect a specimen with more than one detection channel, to detect defects on the specimen by separately processing the data acquired by each of the channels, and to classify the defects by separately processing the data acquired by each of the channels. The defects detected by each of the individual channels may also be further processed separately, for example, by generating different wafer maps, each illustrating the defects detected by only one of the individual channels. The results generated by more than one channel of such a system may then be combined using, for example, Venn addition of the individual wafer maps.
Additional examples of systems and methods for classifying anomalies on sample surfaces are illustrated in U.S. Pat. No. 6,590,645 to Chen et al., U.S. Pat. No. 7,016,031 to Chen et al., and U.S. Pat. No. 7,038,772 to Chen et al., which are incorporated by reference as if fully set forth herein. In these systems and methods, two or more defect maps may be provided for the same sample surface at different detection sensitivities and/or processing thresholds. The defect maps may then be compared for better characterization of the anomalies as scratches, area anomalies, or point anomalies. The results from such map(s) can be used to monitor the process conditions to obtain better yield.
The systems and methods described in the above-referenced patents have provided significant advantages for classifying anomalies. Nevertheless, the systems and methods could be improved in a number of ways. For instance, in the systems and methods described in the above-referenced patents, two or more maps of defects detected at different detection sensitivities on a wafer can be used to distinguish the defect types by size, which is advantageous because relatively large size defects will impact the yield of semiconductor devices for a given design dimension. However, this method of finding defects by size is not sufficient to separate all killer defect types from each other and to separate killer defect types that are related to various wafer manufacturing processes.
Examples of systems and methods for a wafer inspection system using multiple angles and multiple wavelength illumination are illustrated in U.S. Pat. No. 6,956,644 to Biellak et al., which is incorporated by reference as if fully set forth herein. As described in this patent, a method for detecting an anomaly on a top surface of a substrate includes directing a first radiation beam having a first wavelength at the top surface of the substrate at a first angle measured from normal and directing a second radiation beam having a second wavelength at the top surface of the substrate at a second angle measured from normal, where the second wavelength is not equal to the first wavelength. The method then includes detecting scattered radiation from the first radiation beam and the second radiation beam to detect the presence of particles or crystal-originated pits (COPs) and to differentiate between the two. Differences in the scattered radiation detected from the first radiation beam and from the second radiation beam provide the data needed to differentiate between particles and COPs.
The systems and methods described in the above-referenced patent have provided significant advantages for wafer inspection. Nevertheless, the systems and methods could be improved in a number of ways. For instance, the methods illuminate wafers at two oblique angles of incidence and different wavelengths. The methods differentiate between COPs and particles using the scattering signal and strength. But this method may be limited to differentiating between COPs and particles. Therefore, this method may not be able to address needs of other defect type classification.
Examples of systems and methods for simultaneous or sequential multi-perspective specimen defect inspection are illustrated in U.S. Pat. No. 6,922,236 to Vaez-Iravani et al., which is incorporated by reference as if fully set forth herein. In this patent, systems and methods for inspecting a surface of a specimen such as a semiconductor wafer are provided. A system may include an illumination system configured to direct a first beam of light to a surface of the specimen at an oblique angle of incidence and to direct a second beam of light to a surface of the specimen at a substantially normal angle. The system may also include a collection system configured to collect at least a portion of the first and second beams of light returned from the surface of the specimen.
The systems and methods described in the above-referenced patent have provided significant advantages for defect inspection. Nevertheless, the systems and methods could be improved in a number of ways. For instance, the defect inspection system for semiconductor wafers described in this patent includes an illumination module with oblique and normal angles and a signal collection module. The patent addresses the hardware design of a defect inspection system, but does not include methodology of how to conduct defect type classification.
Accordingly, it would be advantageous to develop methods and systems for identifying defect types on a wafer that do not have one or more of the disadvantages of the methods and systems described above.